Selective activation of β3-adrenoceptors by octopamine:: comparative studies in mammalian fat cells

Numerous synthetic agonists selectively stimulate beta(3)-adrenoceptors (ARs). The endogenous catecholamines, noradrenaline and adrenaline, however, stimulate all the beta-AR subtypes, and no selective physiological agonist for beta(3)-ARs has been described so far. The aim of this study was to investigate whether any naturally occurring amine can stimulate selectively beta(3)-ARs. Since activation of lipolysis is a well-known beta-adrenergic function, the efficacy and potency of various biogenic amines were compared with those of noradrenaline, isoprenaline, and beta(3)-AR agonists 4-(-{[2-hydroxy-(3-chlorophenyl)ethyl]-amino}propyl)phenoxyacetate (BRL 37,344) and (R,R)-5-(2-{[2-(3-chlorophenyl)-2-hydroxyethyl]-amino}propyl)-1,3-benzodioxole-2,2-dicarboxylate (CL 316,243) by testing their lipolytic action in white fat cells. Five mammalian species were studied: rat, hamster and dog, in which selective beta(3)-AR agonists act as full lipolytic agents, and guinea-pigs and humans, in which beta(3)-AR agonists are less potent activators of lipolysis. Several biogenic amines were inefficient (e.g. dopamine, tyramine and beta-phenylethylamine) while others (synephrine, phenylethanolamine, epinine) were partially active in stimulating lipolysis in all species studied. Their actions were inhibited by all the beta-AR antagonists tested, including those selective for beta(1)- or beta(2)-ARs. Octopamine was the only amine fully stimulating lipolysis in rat, hamster and dog fat cells, while inefficient in guinea-pig or human fat cells, like the beta(3)-AR agonists. Ln rat white fat cells, beta-AR antagonists inhibited the lipolytic effect of octopamine with a relative order of potency very similar to that observed against CL 316,243. Competitive antagonism of octopamine effect resulted in the following apparent pA(2) [-log(IC50), where IC50 is the antagonist concentration eliciting half-maximal inhibition] values: 7.77 (bupranolol), 6.48 [3-(2-ethylphenoxy)-1[(1 S)-1,2,3,4-tetrahydronaphth-1-ylaminol](2S)2-propanol oxalate, SR 59230A, a beta(3)-selective antagonist], 6.30 [erythro-D,L-1 (7-lethylindan-4-yloxy)-3-isopropylamino-butan-2-ol, ICI 118,551, a beta(2)-selective antagonist] and 4.71 [(+/-)-[2-(3-carbomyl-4-hydroxyphenoxy)ethylamino]-3-[4-(1 -methyl-4-trifluoromethyl-2-imidazolyl)-phenoxy]2-propanolmethane sulphonate, CGP 20712A, a beta(1)-selective antagonist],octopamine had other properties in common with beta(3)-AR agonists: stimulation of oxygen consumption in rat brown fat cells and very low affinity in displacing [H-3]CGP 12,177 binding to beta(1)- or beta(2)-ARs in dog and rat adipocyte membranes. In Chinese hamster ovary (CHO) cells expressing human beta(3)-ARS octopamine inhibited [I-125]ICYP binding with only twofold less affinity than noradrenaline while it exhibited an affinity around 200-fold lower than noradrenaline in CHO cells expressing human beta(1)- or beta(2)-ARs. These data suggest that, among the biogenic amines metabolically related to catecholamines, octopamine can be considered as the most selective for beta(3)-ARs.